Interface properties probed by active THz surface emission in

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Surfaces, Interfaces, and Applications

Interface properties probed by active THz surface emission in graphene/SiO/Si heterostructures 2

Zehan Yao, Lipeng Zhu, Yuanyuan Huang, Longhui Zhang, Wanyi Du, Zhen Lei, Ajay Soni, and Xin Long Xu ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b11301 • Publication Date (Web): 25 Sep 2018 Downloaded from http://pubs.acs.org on September 26, 2018

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ACS Applied Materials & Interfaces

Interface properties probed by active THz surface emission in graphene/SiO2/Si heterostructures Zehan Yao†, Lipeng Zhu†, Yuanyuan Huang†, Longhui Zhang†, Wanyi Du†, Zhen Lei†, Ajay Soni‡, Xinlong Xu†,* †

Shaanxi Joint Lab of Graphene, State Key Lab Incubation Base of Photoelectric Technology

and Functional Materials, International Collaborative Center on Photoelectric Technology and Nano Functional Materials, Institute of Photonics & Photon-Technology, Northwest University, Xi'an 710069, China. ‡

School of Basic Sciences, Indian Institute of Technology, Mandi, H.P. 175005, India

Abstract Graphene/semiconductor heterostructures demonstrate the improvement of traditional electronic and optoelectronic devices due to their outstanding charge transport properties inside and at the interfaces. However, very limited information has been accessed from the interfacial properties by traditional measurement. Herein, we present an active THz surface emission spectroscopy for the interface build-in potential and charge detrapping time constant evaluation from interface of graphene on SiO2/Si (Gr/SiO2/Si). The active THz generation presents an intuitive insight into the depletion case, weak inversion case, and strong inversion case at the interface in the heterostructure. By analyzing the interface electric field induced optical rectification (EFIOR) in a strong inversion case, the intrinsic build-in potential is identified as -0.15 V at Gr/SiO2/Si interface. The interface depletion layer presents 44% positive THz intrinsic modulation by the reverse gate voltage and 70% negative THz intrinsic modulation by the forward gate voltage. Moreover, a time dependent THz generation measurement has been used to deduce the charge detrapping decay time constant. The investigation will not only highlight the THz surface emission spectroscopy for the graphene-based interface analysis, but also hold a potential for the efficient THz intrinsic modulation as well as the enhancement of THz emission by the heterostructures.

Key words: Graphene, THz emission, silicon, interface, modulation

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1. INTRODUCTION Nobel laureates Kroemer said “the interface is the device”, as the interface properties control the dynamics of carriers (electrons and holes) as well as the elementary excitation (i.e., phonons, excitons, and plasmons) in electronic and optoelectronic devices. Optical probing is an important nondestructive method, which has been developed for studying surfaces and interfaces such as second-harmonic and sum-frequency generation spectroscopy in 1980s1. Femtosecond laser induced THz emission spectroscopy has also been demonstrated as a sensitive and contactless method in understanding the surfaces2-3 and interfaces4 of semiconductors, even though it is still on the development stage. When a femtosecond laser interacts with the semiconductors, the transient carriers, dipoles, and other quasi-particles can emerge at the surfaces and interfaces. Under the depletion field or gating field, the transient carriers or nonlinear dipoles can generate THz pulses. These THz pulses carry valuable parameters such as amplitude, phase, and polarization, which are related to the carrier distribution, depletion field, dipole orientation, carrier concentration and trapping near the surface and interface. As such, THz emission spectroscopy has been employed for integrated circuit inspection5, molecular dynamics at the surface6, as well as the surface property of oxidized silicon7. Graphene, as a typical two-dimensional (2D) layered material, demonstrates fascinating surface and interface properties when in contact with other 2D and 3D materials due to the weak van Der Waals interaction. The surface of graphene can reach atomic flatness without dangling bond and strain, thus does not require the lattice matching8-9. Comparatively a metal layer thinner than 1 nm mainly formed by metallic particles instead of continuous film may introduces a plasmonic effect as dominant contribution in metal/semiconductor heterostructure instead of interface effect10. As compared to other 2D layered materials, the high carrier mobility and


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ambient stability render graphene preferable to be an ultrathin transparent gate electrode. Graphene/semiconductor heterostructures have been used to improve the performance of traditional devices. For example, Graphene/semiconductor has been demonstrated in ultrasensitive photodetectors11-14, high efficient solar cells15-17, high sensitive chemical sensors1821

as well as the position sensitive detectors22-23. In these applications, information such as

interface build-in potential, charge separation, carrier recombination and trapping at the graphene/semiconductor interface is desirable. Interface build-in potential determined by energy band bending plays a key role in understanding the physical process in graphene based heterostructure. Methods for evaluating the interface build-in potential have been realized by X-ray photoelectron spectroscopy and capacitance (C-V) measurement24-26. However, X-ray photoelectron spectroscopy is invasive for the devices. CV measurement is unavailable for the localized detection and can only be suitable for heterostructures based on low resistivity substrate with high doping concentration, which cannot be applied in numerous application fields such as THz devices. For example, low doped silicon is usually employed for THz modulation based on graphene/Si hybrid diode, which show the intensity modulation due to the interface effect27-28. However, only maximum of 18% transmission difference between graphene/Si and silicon substrate call for deep interfacial study of the graphene/Si heterostructures27. The interfacial information can be further used to improve the THz modulation depth by the interfacial effect, for which localized interface depletion electric field and carrier dynamics are needed. In this paper, we present an efficient and noninvasive method for evaluating the interface build-in potential and trapped charge dynamics of graphene on SiO2/Si (Gr/SiO2/Si) by active THz generation at the interface. The interface electric field induced optical rectification (EFIOR)


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provides a relationship between interface build-in potential and THz generation. Active THz generation presents an intuitive insight into the depletion case, weak inversion case, and strong inversion case at the interface. The intrinsic build-in potential at Gr/SiO2/Si interface can be identified as -0.15 V by analyzing the EFIOR effect in a strong inversion case. Modification of the interface depletion layer present 44% positive and 70% negative THz intrinsic modulation by reverse and forward gate voltage, respectively. Moreover, the charge detrapping decay time constant can be obtained by a time dependent THz generation measurement. The results are very useful for the THz surface emission spectroscopy on the graphene-based interface analysis and pave the way for the efficient THz intrinsic modulation as well as the enhancement of THz emission from the heterostructures. 2. EXPERIMENTAL SETUP Graphene used in our experiment was grown on copper substrate by atmospheric pressure chemical vapor deposition (APCVD) method29-30. We employed 930 sccm CH4 as carbon precursor, 1.6 sccm H2 and 45 sccm Ar as assist gas. The growth process for graphene was carried out for 12 minutes at 1000ºC. After CVD process, the graphene was transferred onto 1 mm thick n-type polished Si (100) substrate with resistance ρ>5000 Ω cm-1 by dissolving the Cu foil in the etchant (1 g ferrite nitrate and 1 mL hydrochloric acid in 25 mL deionized water)31. After the transfer process, two metal rings were patterned closely to graphene and Si as gate electrodes as shown in Fig. 1a. Thickness of the oxide layer is ~2 nm measured by an ellipsometer (Semilab company) with the accuracy ±5 Å. The graphene film was characterized by Raman spectrum and visible-near infrared spectrum as shown in Fig. S1. The G peak at 1580 cm-1 is from the E2g vibration mode of sp2 carbon (Fig. S1(a) supporting information). Visiblenear infrared spectrum (Fig. S1(b) supporting information) and the SEM image (Fig. S1(c)


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supporting information) suggest a bilayer graphene and a related well-formed Gr/SiO2/Si interface. The femtosecond excitation laser pulse with 35 fs duration, 800 nm center wavelength, and 1 kHz repetition rate were generated from a Ti:sapphire regenerative amplifier (Spectra-Physics Spitfire). The schematic of the experimental setup is shown in Fig.1b. The polarization of the incident laser beam was fixed at s-polarized by a half-wave plate. The THz generation is taken in a reflection configuration with an angle of 45°. The excitation laser beam was fixed at 50 mW with a diameter of 3 mm on the sample and was blocked by a telfon-silicon wafer to get rid of the residual excitation laser. There is no evidence of THz radiation from graphene due to the photon drag effect as the low pump intensity32-33. A couple of wire-grid polarizers (WGP) P1 and P2 (Fig.1b) were used to get the polarization information of the generated THz wave. The ppolarized THz radiation was polarized along the y-z plane (Fig.1a insert), which can be extracted with a perpendicularly aligned WGP (P1 in Fig. 1b). The s-polarized THz radiation was polarized along the x axis and parallel to the sample surface (Fig.1a insert), which was obtained after adding another 45° WGP (P2 in Fig. 1b)3. The THz signal was detected with a 2 mm ZnTe crystal (110) combined with a balanced detector, while the probe beam was kept in p-polarized. More details can be found in our earlier reports3.


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Fig. 1 Schematic of experimental setup, (a) Illustration of Gr/SiO2/Si sample, (b) Schematic of THz surface emission spectroscopy system in a reflection configuration. Here, P1 and P2 represent THz wire-grid polarizers and HWP refers to the half-wave plate. 3. RESULTS AND DISCUSSION The comparison of p-polarized THz generation from the Gr/SiO2/Si and Si (Fig. 2a) shows that THz generation from the Gr/SiO2/Si has enhanced, considerably. Additionally, the related frequency-domain signals (Fig. 2b) indicate that the enhancement of THz radiation is within a broadband region. Here, the pump power was insufficient to generate THz radiation from graphene, and the penetration depth of 800 nm pump laser on Si is on ten micrometer length scale (deduced by the absorption coefficient34 of 8.5×102 cm-1), which exclude the THz generation from Si bulk. The observations suggest the enhancement of THz generation is mainly coming from the photocarriers in the interface layer with the picosecond response. Fig. 2c demonstrates the azimuthal dependence of THz radiation at two different polarization states. The s-polarized THz radiation from Gr/SiO2/Si under oblique incidence was negligible compared with the p-polarized THz radiation; on the other hand, there was no evidence of THz generation


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under normal incidence in a transmission configuration (Fig. S2 supporting information). These results indicate that the electric component of the THz radiation parallel to the sample surface is expected to be negligible. As such, the electric component of THz radiation due to the interfacial effect by the build-in potential is normal to the sample surface.

Fig. 2 Comparison of THz generation by Gr/SiO2/Si and Si without applying gate voltage. (a) P-polarized THz generation in time domain and (b) in frequency domain. The azimuthal angle was fixed at position C in Fig. 2(c). (c) Comparison of p- and s- polarized THz peak-tovalley amplitude generated by Gr/SiO2/Si with respect to the sample azimuthal angle. The position A, B, and C correspond to three azimuthal angles related to the minimum, middle, and maximum amplitude of THz generation. (d) Comparison of p-polarized THz generation with respect to the sample azimuthal angle. The square and circular dots represent the experimental


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results. The solid curves represent the fitting results. The dash curves represent the THz generation from transient photocurrent effect. Second order nonlinear polarization and transient photocurrent are expected to generate THz waves at the surface and interface, which can be described by35:

E

THz

∂ 2 P ∂j ∝ ∫ ( 2 + )dz , ∂t ∂t

(1)

where P is the nonlinear polarization from the dipole oscillation and j is the transient photocurrent from the photocarrier drift and diffusion. Optical rectification dominates the nonlinear polarization effect, which is proportional to the second-order nonlinear susceptibility χ(2) and can be expressed as:

P (2) (Ω) = ε 0 χ (2) E (ω + Ω) E * (ω )

(2)

where Ej and Ek correspond to the electric field of excitation wave; ω is the frequency of femtosecond incident laser; Ω is the frequency of generated THz wave; ε0 is the vacuum permittivity. The second order nonlinear effect cannot happen in the centrosymmetric crystal such as Si as χ(2)=0 (Equation (2)). However, the centrosymmetry would be broken at the interface depletion layer by the build-in potential φd, leading to a nonzero effective χ(2). In this case, the effective second-order nonlinear susceptibility in Equation (2) can be described by36:

χ eff( 2) (0; ω , −ω ) = χ (3) (0; ω , −ω ) E DC ( z ) ,

(3)

where χ(3) is the third-order nonlinear susceptibility and EDC is the DC electric field in the depletion layer. The related nonlinear effect is considered as electric field induced optical


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rectification (EFIOR)36. Since the DC electric field varies along the penetration depth of the pump laser, the effective nonlinear polarization can be given by37:

Peff(2) (Ω) = ε 0 χ (3) E (ω + Ω) E * (ω ) ∫ EDC ( z )dz ∝ ϕ d .

(4)

This indicates that the amplitude of THz generation from EFIOR effect is proportional to φd. The Fermi level mismatch in the Gr/SiO2/Si heterostructure results in an energy band bending and increases the φd at the interface38 as shown in Fig. S3 (supporting information). Moreover, the increase of φd would result in enhancement of transient photocurrent effect7. Therefore, graphene coating would lead to an enhanced THz generation from both EFIOR effect and transient photocurrent effect. The azimuthal angle dependence of THz generation from Gr/SiO2/Si and Si are shown in Fig. 2d. THz amplitude presents the maximum values at 0° and 90°, while the minimum values at 45°and 135°. Thus, the THz generation exhibit 2-fold rotation symmetry with the change of azimuthal angle within 180°. The azimuthal angle dependent THz generation is due to the EFIOR effect36, while the azimuthal angle independent THz generation (indicated by the transverse dash curves in Fig. 2d) is from the transient photocurrent effect39. Compared with the Si, transient photocurrent in Gr/SiO2/Si has been enhanced with a significant upward shifting (Fig. 2d). We can fit the azimuthal angle dependence of the THz generation in Fig. 2d by the following formula40:

E THz = m sin 4θ + n ,

(5)

Where m corresponds to the magnitude of EFIOR depending on the sample azimuthal angle θ, and n is related to the transient photocurrent, which is independent of θ. Thus, we can evaluate


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the THz enhancement due to the EFIOR effect and transient photocurrent effect from the parameters m and n. The enhanced factors are calculated by dividing the m or n value in Gr/SiO2/Si by that in bare Si. As shown in Table 1, the enhanced factor from EFIOR due to φd after graphene coating is approximately 2.2, while the enhanced factor from the transient photocurrent effect is approximately 1.51. These results suggest that graphene coating have significant effect on both the nonlinear polarization and the transient carrier dynamics, indicating the capability of THz radiation enhancement by the interface engineering. Table 1 Fitting parameters for Fig. 2d by Equation (5).

Bare Si

Gr/SiO2/Si

Enhanced factor

m : EFIOR

0.06

0.11

2.2

n : Current

0.37

0.56

1.51


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Fig. 3 Gate voltage response of THz generation from Gr/SiO2/Si heterostructure. (a) Generated THz time domain signals from Gr/SiO2/Si at -2 V, 0 V, 2 V, and 4 V, respectively. (b) Gate voltage dependence of THz peak-to-valley amplitude at A, B, and C positions. (c-e) The Gr/SiO2/Si interface stays in (c) depletion case in region I and region II, (d) weak inversion case in region III, and (e) strong inversion case in region IV. The carrier dynamics at the Gr/SiO2/Si interface can be tuned by the gate voltage (Vg), which introduces an active THz generation by the interface engineering. As shown in Fig. 3a, THz amplitude under -3 V is three times larger than that under 3 V. The related signal in frequency domain is shown in Fig. S4 (supporting information), which suggests that the band-width does not change, representing the photocarrier dynamics is almost the same for broadband modulation. We extract three azimuthal angles related to the minimum, middle, and maximum


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THz peak-to-valley amplitude, labeled as A, B, and C in Fig. 2c and Fig. S5a (supporting information). At B position, the THz amplitude is equal to the azimuthal angle independent THz generation, indicating that THz radiation contributed from EFIOR effect is small enough and transient photocurrent effect dominates the THz generation. The contribution of EFIOR effect reaches maximum values at A and C with the THz time-domain signals shown in Fig. S5b (supporting information). The gate response of THz amplitude at A, B, and C is shown in Fig. 3b. The extracted THz modulation depths indicate 44% positive intrinsic modulation depth under 3 V, and 70% negative THz intrinsic modulation depth under -3 V at all three positions as shown in Fig. S5c (supporting information). The 70% negative intrinsic THz modulation is comparable to the extrinsic THz modulation28. These results suggest an active interfacial method for THz intrinsic modulation instead of THz extrinsic modulation. Since the THz generation from Gr/SiO2/Si is dominated by φd, the Vg dependent THz generation is studied according to the interface energy band structure. As shown in Fig. 3b, the gate response of THz generation can be divided into four regions. In region I, THz generation is almost independent to the forward Vg when the Vg is larger than 3 V. In this case, the interface layer approaches the flat band state. However, due to the high breakover current, Si bulk resistance eliminates the bias effect on the interface layer4. In region II, THz generation increases linear with the Vg from 3 V to -1 V. In this case, the interface layer stays in the depletion case. The majority carriers are depleted and the minority carriers are negligible at the Si surface; i.e., EF on Si surface is higher than the intrinsic energy level in bulk Ei as shown in Fig. 3c. This interface depletion layer can be regarded as a capacitance, in which Vg would introduce the shifting of φd due to the shifting of EF on graphene and Si surface. Under reverse Vg> -1 V, the modulation of THz generation reaches saturation. In previous


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study, similar saturation under reverse Vg from metal/semiconductor interface is attributed to the avalanche breakdown effect4. In our experiment, avalanche breakdown current has not been observed in the I-V measurement as shown in Fig. S6 (supporting information). Therefore, we attributed this saturation to the interface inversion case. In region III in Fig. 3b, active THz generation has a saturation trend when increasing the reverse Vg from -1 V to -3 V. In this case, minority carriers accumulate on Si surface, and the density cannot be neglected compared with the majority carrier density in Si bulk; i.e., EF is lower than Ei at the interface as shown in Fig. 3d. The interface layer is in a weak inversion case and can be divided into a depletion layer and an inversion layer within 0-10 nm. The inversion layer can be regarded as another capacitance, which screens the Vg and lead to a saturation of φd. In the inversion case, an insulating layer stays between graphene and Si and acts as a capacitor to store the large accumulating charges at the inversion layer and graphene. Since the graphene is thinner than 1 nm and the inversion layer on Si surface is of 1-10 nm, the only 2 nm native oxide layer on Si surface is still sufficient for the formation of inversion layer. In region IV, THz generation is independent to the reverse Vg for Vg< -3 V. In this case, the minority carrier density at the Si surface surpasses the majority carrier density in bulk. The interface layer is in a strong inversion case, where the inversion layer completely screen the Vg, and φd reaches a maximum constant value when φdm = 2φb, where φb = Ei-EF, as shown in Fig. 3e.


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Fig. 4 Interface φd versus Vg. (a) The φd as a function of Vg. (b) THz generation and calculated C-V curve as a function of Vg. In order to evaluate the gate response, we extracted the maximum φd in the strong inversion case by the following formula24:

ϕd m = −

2k0T N ln( D ) , e N0

(6)

where temperature is T= 300 K; the intrinsic carrier concentration is N0 = 1×1010 cm-3; the doping donor concentration is ND=8×1011cm-3. From Equation (6), the saturation φd at Gr/SiO2/Si interface can be calculated as φdm=-0.22 V. In order to get the relationship between Vg and φd, THz generation due to the pure EFIOR is calculated by A EFIOR = ( Amax − Amin ) 2 (where Amax and Amin are the values at C and A positions in Fig.3b) as shown in Fig. S7 (supporting information). Considering φdm = -0.22 V under reverse Vg for the strong inversion case, φd under different Vg can be extracted as shown in Fig. 4a. The intrinsic φd at Gr/SiO2/Si interface without applying Vg is φd0=-0.15 V as shown in Fig. 4a, smaller than that from Si with higher doping concentration38. Moreover, capacitance (under high-frequency condition) of interface depletion layer can be calculated by24 C =

N d eε oxide ε 0 2 ϕ d , where εoxide=3.9 is the permittivity of SiO2 layer. As


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shown in Fig. 4b, the gate behavior is opposite to the THz generation, consistent with the C-V measurement on the n-doped Si surface7. These results suggest that active THz generation is an effective method for probing the graphene based interface build-in potential. Interface carriers can be used for external THz modulation, for which part of the carrier information can be deduced based on the multilayer calculation28, 41. However, active THz generation refer to an intrinsic modulation based on the THz emission from carrier dynamics, which is totally different from the extrinsic modulation. Since THz generation from our semiconductor heterostructure is directly related to the interface build-in potential, this intrinsic THz modulation can evaluate the build-in potential, which is unavailable in external THz modulation yet. Moreover, carrier dynamics such as charge trapping effect can also be probed in active THz generation.

Fig. 5 THz generation as a function of time with switching Vg on/off. (a) The gate response of


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THz generation at 2 V (black curve) and 4 V (red curve). (b) The gate response of THz generation at -2 V (black curve) and -1 V (red curve). (c) The long time decay of THz generation at reverse Vg. (d) The fitting results for the detrapping process after switching off the reverse Vg. Charge trapping effect at the interface layer can prolong the carrier relaxation time and show persistent photoconductivity in graphene based devices and materials42-43. Charge trapping effect in the THz generation can be further observed by introducing a time dependent THz measurement at different Vg. The peak value of the THz time domain signal as a function of time (Fig.5) is recorded by a lock-in amplifier with a time constant 300 ms. The azimuthal angle of the sample is fixed at position B (Fig. 2c) to minimize the EFIOR effect. As shown in Fig. 5a, the THz generation exhibits a rapid response when switching on and off the forward Vg. However, additional long time decay can be observed when switching off the reverse Vg as shown in Fig. 5b. To further understanding this effect, we take the long time dependent THz generation under different reverse Vg in the strong inversion case as shown in Fig.5c. It is evident that with the decrease of the Vg, the decay time increases appreciably. By considering the Gr/SiO2/Si as a series RC electronic system, decay time constant can be expressed as τ = RC ox . In our experiment, τ is about several tens of seconds, the resistance of the interface R is smaller than 5000 Ω, and thus the capacitance of SiO2 Cox is larger than 2 mF. By considering the inversion layer as a plane-parallel capacitor, the capacitance of SiO2 can be expressed as

C ox = ε ox ε 0 S d , where εox=3.9 is the dielectric constant of SiO2, S=7 mm2 is the spot size and d=2 nm is the thickness of SiO2. Thus the related Cox is 12 µF, which is inconsistent with the calculated result from the RC model. Hence, this long time decay cannot be simply regarded as a RC charge-discharge process. As shown in Fig. S8 (in the supplementary information), THz generation is directly proportional to the φd, while the φd has a relationship to the accumulating


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non-equilibrium carriers at the interface. Therefore, the THz generation decay after switching off the reverse Vg can directly reflect the dynamics of the non-equilibrium carriers at the interface. The decay of non-equilibrium carriers can be fitted by the following formula:

n(t ) = [ n1 exp(− t τ 1 ) + n2 exp(− t τ 2 )] ⊗ G (t ) ,

(7)

where n1 and n2 refer to the trapped carrier density at the SiO2/Si interface and graphene/Si interface, respectively; τ1 and τ2 are related to the decay time constant of the trapped carriers. A convolution process is used to exclude the system response, where G(t) is the Gauss system response function with a half width full maximum of 1 s. The fitting parameters are shown in Table 2. At -2 V Vg, the detrapping time constant for charges at SiO2/Si interface is 1.2 s, and it is shortened to 0.83 s at -5 V Vg. While the detrapping time constant for charges at graphene/SiO2 interface is 28 s, and prolong to 41 s at -5 V Vg.

Table 2 Fitting parameters from Equation (7). n1 and n2 refer to the two types of charge traps respectively, and τ1 and τ2 are decay time constant.

Vg (V)

n1(a.u.)

τ1 (s)

n2(a.u.)

τ2 (s)

-2 V

2

1.2

0.19

28

-3 V

2.4

1

0.24

30

-5 V

2.22

0.83

0.41

41

In order to screen the external reverse Vg, large amounts of holes can accumulate at the strong inversion layer and electrons can accumulate at graphene, which will give rise to the EF of graphene as shown in Fig. 4c. In the Gr/SiO2/Si heterostructure, charge trapping sites exist at the graphene/SiO2 and SiO2/Si interface42. When switching off the reverse Vg, the accumulating


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carriers in graphene and the excess holes at Si surface recovers within picosecond44 and microsecond45 in time scale, respectively. Graphene Fermi level recovers and φd decreases rapidly along with the fast carrier dynamics as shown in Fig. 6a. The trapped charges at SiO2/Si interface recover via recombination with carriers at the Si surface and the trapped charges at graphene/SiO2 interface recombine with holes in the graphene or by tunneling into the Si. The delay time constant of non-equilibrium detrapping process is on the scale of several seconds or tens of seconds43, 46, leading to a non-equilibrium φd as shown in Fig. 6b. These results reveal the impact of charge trapping effect on the THz generation in the Gr/SiO2/Si heterostructure and suggest THz surface emission spectroscopy as a useful tool for probing the interface carrier dynamics.

Fig. 6 Band structure of Gr/SiO2/Si interface after switching off the reverse Vg. (a) The fast carrier recombination process within picoseconds in graphene and microseconds in Si, and (b) the slow detrapping process of several seconds or tens of seconds. 4. CONCLUSIONS In summary, we have presented a simple and efficient active THz surface generation method


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for evaluating interface build-in potential and probing the charge detrapping dynamics in the graphene/semiconductor heterostructures. By modifying the interface depletion layer, THz generation can be tuned by 44% under reverse Vg and 70% under forward Vg. Understanding of this intrinsic THz modulation based on interface band structure provide an intuitive insight into the depletion case, weak inversion case, and strong inversion case at Gr/SiO2/Si interface. Theoretical analysis based on the interface EFIOR effect in strong inversion case suggests the 0.15 V intrinsic build-in potential. Moreover, the charge detrapping dynamics can be probed by a time dependent THz generation measurement. Build-in potential is directly related to the carrier distribution, band bending, which determine the basic properties of the interface. Carrier dynamics is the crucial part for high performance optoelectronic and photonic devices. Therefore, our investigation is of great important for the development of graphene-based interface optoelectronic and photonic devices. Moreover, our results also pave the way for the effective intrinsic modulation and enhancement of THz emission from heterostructures. ASSOCIATED CONTENT

Supporting Information Graphene characterization, THz generation in a transmission configuration, Band structure at the Gr/SiO2/Si interface, Active THz generation from Gr/SiO2/Si, I-V measurement for Gr/SiO2/Si, Gate response of THz generation from EFIOR effect, THz generation versus build-in potential (PDF) AUTHOR INFORMATION

Corresponding Author


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* E-mail: [email protected]. Fax: +86 29 88303336. ACKNOWLEDGMENT This work was supported by National Natural Science Foundation of China (No. 11774288, 11374240), Key Science and Technology Innovation Team Project of Natural Science Foundation of Shaanxi Province (2017KCT-01), Excellent Doctoral Dissertation Training Project of Northwest University (YZZ17101). ABBREVIATIONS EFIOR, electric filed induced optical rectification; Gr/SiO2/Si, graphene/SiO2/Si. REFERENCES (1) Shen Y. R., Surface properties probed by second-harmonic and sum-frequency generation. Nature 1989, 337, 519-525. (2) Zhang X. C.; Hu B. B.; Darrow J. T.; Auston D. H., Generation of femtosecond electromagnetic pulses from semiconductor surfaces. Applied Physics Letters 1990, 56, 1011-1013. (3) Huang Y.; Zhu L.; Zhao Q.; Guo Y.; Ren Z.; Bai J.; Xu X., Surface Optical Rectification from Layered MoS2 Crystal by THz Time-Domain Surface Emission Spectroscopy. ACS Applied Materials & Interfaces 2017, 9, 4956-4965. (4) Zhang X. C.; Auston D. H., Optoelectronic measurement of semiconductor surfaces and interfaces with femtosecond optics. J. Appl. Phys 1992, 71, 326-338. (5) Kiwa T.; Tonouchi M.; Yamashita M.; Kawase K., Laser terahertz-emission microscope for inspecting electrical faults in integrated circuits. Opt. Lett. 2003, 28, 2058-2060.


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Interface properties probed by active THz surface emission in

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